Ice chunks the size of a Volkswagen falling from the sky! Sounds like a Hollywood special effects scene in an action movie, right? Unfortunately, this is a real-life danger that can occur just about anywhere, but especially on wet stacks running in cold weather.

It should go without saying that ice falling from a tall stack can have damaging, even catastrophic effects on process equipment and personal safety. But with some situational knowledge and attention to design details, you can prevent ice build-up on wet-stacks before it becomes a problem. 


Icing and the Downwash Effect

Essentially, to operate an ice-free wet stack system, you need to properly handle the discharge of wet flue gas during prolonged exposure to cold temperatures. Units running at low loads on cold, windy days can see a dangerous icing develop from an effect called plume downwash.

Plume downwash occurs when a cross-wind at the top of the stack deflects the plume from its vertical path. This phenomenon is more likely to happen when flue gas exits at a lower velocity—like, for example, when units aren’t running at full capacity. As the wind impacts the plume, the plume is pushed downward onto the stack, causing the liquid within the flue gas to deposit on the stack’s surfaces. 


And what happens when moisture is allowed to build up on cold surfaces? You guessed it—ice forms.

The Likely Ice Hangouts

Ultimately, all stacks can experience downwash if wind speeds are high enough. The only questions are:

  • Under what conditions and how frequently will downwash occur, based on the expected boiler load schedule and wind frequency profile?
  • How much downwash is considered acceptable? 

Thankfully, much of the guesswork can be eliminated by using computational fluid dynamic (CFD) modeling. CFD modeling is extremely well-suited to simulate the stack plume over a range of plant operating and atmospheric conditions to predict the potential for plume downwash. And if you haven't already identified icing with the naked eye, CFD simulations can be used to predict not only if icing can occur, but where it can form on the stack.

If conditions are right for plume downwash, the following areas are most likely to experience problems, including potential ice-buildup:

  • The stack hood
  • The liner extension for single or multiple liners
  • The stack roof
  • Metal railings
  • Metal platforms
  • The stack shell 


These areas are exposed directly to plume downwash, and therefore, icing in the right conditions— some more so than others. Heat tracing is often recommended for some of these surfaces to eliminate snow accumulation and excessive ice build-up, but care should be taken to ensure the drainage run-off doesn’t create a secondary icing problem.

More details about the icing potential in these areas can be found in the EPRI Revised Wet Stack Design Guideline, section 1.4.9.

Wet Stack Design Considerations

According to the EPRI Revised Wet Stack Design Guideline, the potential for icing can be reduced by employing the following steps:

  1. Install a liner extension above the shell to minimize the potential for plume downwash.
  2. Select a stack-liner discharge velocity that minimizes plume downwash over the expected operating range of the unit at the existing local wind conditions and that is consistent with other design objectives.
  3. During cold ambient temperature conditions with high winds, run the unit at near full load. (Employing this operating procedure may be natural under such conditions, because more power is consumed in below-freezing weather.)
  4. Use heated annulus air or heat-tracing to heat the stack hood, roof, or other areas where ice forms on the top of the stack above the freezing point.

Any uncertainty in any of these recommendations can be discussed with our Gas Flow and Wet-Stack Design experts.


Icing can occur at below freezing conditions all winter long, every winter, creating potentially dangerous conditions for both people and property. If you're running a wet stack at a low load in cold, windy weather, icing is probably going to be a concern for you. Contact us for details and recommendations in order to ensure proper performance and reliable operation of your wet stack located downstream of a wet flue gas desulfurization system (WFGD).

And in any instances where ice is present, be careful!

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